33Impacts of seaborne trade on coal importing countries – global summaryImpacts of changing the quality of coal due to importing
One of the most important aspects of moisture content is the effect on NOx production. Research byIkeda and others (2003) in Japan raised concerns about the impact of the increasing use of Indonesiansubbituminous coals in boilers designed with advanced low NOx burners. They found thatsubbituminous blends with bituminous coals showed evidence of elevated unburnt carbon in the ashand higher NOx production, based on experimentation in a pulverised fuel test furnace with a fuel rateof just 0.1 t/h. The results showed that the blend of coals produced more undesirable effects than wheneach coal was burned separately; the use of the blend resulted in increased NOx emissions andunburnt carbon in fly ash as the concentration of subbituminous increased. However, adaptation of thesecondary air flow angle reduced these effects to enable a higher proportion of subbituminous coal tobe burned.Most bituminous coals that are traded internationally contain some 12–15% moisture (seeTable 3);moisture levels of some Indonesian coals can be well over 20% (seeTable 4), similar to levels seen inPRB coals in the USA. Consequently, a switch to these low sulphur, but higher moisture coals requirecareful blending and testing in the power stations to determine the correct adaptations to ensure thereis minimal or no loss in efficiency or availability.Gunderson and others (1994, 1996) noted that when some US utilities increased the quantity of PRBcoals, the subbituminous coals tended to suppress the pulveriser mill outlet temperature. Attempts toattenuate the temperature increased the parasitic load on the plant and resulted in derating of the plant.However, there were no detrimental effects on burner flame stability despite the higher reactivity ofthe subbituminous coal. Test burns of blends of subbituminous and bituminous coals at the Gibsonplant, Illinois, also resulted in a reduction in the mill outlet temperature and a loss in boiler efficiency(Meehan and others, 1995).8.3 Chlorine contentA literature study carried out by Tillman and others (2009) established the effects of corrosion and ashdeposition on a boiler when blending high chlorine solid fuels into the normal coal feed. The analysisrelied heavily on papers published throughout the 1990s when much of the material came out. Theconclusions highlighted the wealth of knowledge regarding the effects of chlorine content of solidfuels, its sources, and reactions in pulverised fuel firing. Yet, the research over the years was notconsidered exhaustive with plenty of scope for further work. Chlorine is an increasing problem inpulverised fuel combustion with the rising use of different rank coals, but also with the increasingimportance of cofiring (sustainable) fuels such as biomass.